The present invention relates generally to land-based gas turbines, for example, for electrical power generation, and particularly to internal cooling circuits for the nozzle segments of the gas turbine.
Traditionally, compressor bleed air is extracted from the turbine""s compressor for cooling the turbine blades and nozzles. Diversion of cooling air, however, represents a parasitic loss to turbine efficiency. More recently, advanced gas turbine designs have recognized that the hot gas path flow temperature could exceed the melting temperature of the turbine components, necessitating a different cooling scheme to protect those hot gas path components during operation. Steam as a cooling medium has been recognized as superior to air because steam has a higher heat capacity. A gas turbine employing steam as a cooling medium for the nozzle segments has been proposed, for example, in U.S. Pat. No. 5,674,766 of common assignee herewith.
In the cooling scheme set forth in that patent, the inner and outer walls or bands of the nozzle segments between which the nozzle vanes extend are compartmentalized to provide impingement cooling along the outer and inner walls of the segment. Cooling steam is also provided along the walls of the vanes. To accomplish that, the cooling steam is supplied to a first chamber of the outer wall, where it passes through impingement openings in an impingement plate for impingement cooling the outer wall. The steam is then passed radially inwardly through the first and fifth cavities of each stator vane for flow through inserts in those cavities. The inserts have openings and the steam flows through the openings to impingement cool registering portions of the stator vane walls. The steam then flows into an inner chamber of an inner wall and reverses direction for flow radially outwardly through openings in an impingement plate to impingement cool the inner wall. The spent cooling medium then flows radially outwardly through three intermediate cavities, each having an insert with openings for impingement cooling the adjacent walls of the vane. The spent cooling steam then flows outwardly of the segment.
Additionally, air is supplied to a cavity extending adjacent the trailing edge of the vane for cooling the trailing edge. The air flows past turbulators and exits into the hot gas stream through openings in the trailing edge. While the foregoing described design has many advantages, it is desirable to have a more robust design with reduced casting costs and complexity, as well as a reduced number of inserts.
In accordance with a preferred form of the present invention, a nozzle stage is provided having a cooling circuit, e.g., steam and air, of reduced complexity and cost, while meeting cycle requirements. Particularly, the cooling scheme of the present invention for the nozzle stage includes outer and inner bands with vanes extending therebetween. Similarly as in the above-mentioned patent, the inner and outer bands are compartmentalized for impingement cooling of the walls defining the gas path. The present invention, however, provides a cooling circuit within each vane having a flow pattern significantly different from the flow pattern of the prior patent affording the above-mentioned advantages. The present invention provides first, second, third, fourth and fifth cavities between the inner and outer bands of each vane segment. The cavities in each vane are arranged sequentially in that order from the leading edge to the trailing edge. After impingement cooling the gas path wall of the outer band, steam from the outer band flows generally radially inwardly through inserts in the first and second cavities and through openings in the inserts for impingement cooling the registering wall surfaces of the vane. Steam is also supplied to the fourth cavity for flow radially inwardly. However, the fourth cavity does not have an insert and the walls of the vane defining the fourth cavity are not impingement cooled. Rather, they are convectively cooled. Thus, the cooling medium is supplied the first, second and fourth cavities at a relatively low temperature, affording improved cooling adjacent the leading and trailing edges, the hottest portions of the vanes. The steam flowing into the inner band compartment passes through an impingement plate for impingement cooling of the inner band. Spent cooling steam is supplied to the third vane cavity. An insert in the third cavity has openings for impingement cooling of the registering wall surfaces of the vane. The spent cooling steam then flows outwardly of the third cavity for flow generally radially outwardly of the vane segment. The fifth cavity is air-cooled by compressor bleed air. Turbulators are also disposed in the fifth cavity. However, the fifth cavity is closed and does not exhaust air to the hot gas path stream. Rather, the spent cooling air is exhausted into the wheelspace.
In a preferred embodiment according to the present invention, there is provided a turbine vane segment, comprising inner and outer bands spaced from one another and having inner and outer walls, respectively, in part defining a gas path through the turbine, a vane extending in the gas path between the inner and outer bands and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium, a cooling medium inlet for the segment for enabling passage of the cooling medium into a compartment of the outer wall, the cavities including first, second, third, fourth and fifth cavities in sequential order from the leading edge toward the trailing edge, the vane having openings in communication with the compartment and the first, second and fourth cavities to enable passage of the cooling medium from the compartment into the first, second and fourth cavities for flow in a generally radially inward direction along the first, second and fourth cavities, the vane having openings in communication between a compartment of the inner wall and the first, second and fourth cavities for flowing the cooling medium from the first, second and fourth cavities into the compartment of the inner band, the vane having an opening in communication with the compartment of the inner band and the third cavity for flowing the cooling medium generally radially outwardly through the third cavity and outwardly of the vane segment.
In a further preferred embodiment according to the present invention, there is provided a turbine vane segment, comprising inner and outer bands spaced from one another and having inner and outer walls, respectively, in part defining a gas path through the turbine, a vane extending in the gas path between the inner and outer bands and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium, a first cover for the outer band spaced outwardly of the outer wall, a first impingement plate between the first cover and the outer wall in part defining outer and inner chambers on opposite sides of the impingement plate, a cooling medium inlet for the segment for enabling passage of the cooling medium into the outer chamber, the impingement plate having openings for flowing the cooling medium from the outer chamber into the inner chamber through the openings for impingement cooling of the outer wall, the cavities including first, second, third, fourth and fifth cavities in sequential order from the leading edge toward the trailing edge, the vane having openings in communication with the inner chamber and the first, second and fourth cavities to enable passage of the cooling medium from the inner chamber into the first, second and fourth cavities for flow in a generally radially inward direction along the first, second and fourth cavities, a second cover for the inner band spaced inwardly from the inner wall, a second impingement plate between the second cover and the inner wall in part defining outer and inner chambers on opposite sides of the second impingement plate, the vane having openings in communication with the inner chamber of the inner wall and the first, second and fourth cavities for flowing the cooling medium from the first, second and fourth cavities into the inner chamber of the inner band, the second impingement plate having openings for flowing the cooling medium from the inner chamber of the inner band through the openings of the second impingement plate into the outer chamber of the inner band for impingement cooling the inner wall, the vane having an opening in communication with the outer chamber of the inner band and the third cavity for flowing the cooling medium generally radially outwardly through the third cavity and outwardly of the vane segment.